Firestorm duration 180 seconds, When healing, 1st tick kicks in at 20 seconds, thereafter every 10 seconds. So the theoretical maximum MP recovered a Scholar can get from casting firestorm then sitting down to healed is 180 -20 = 160 (for the 1st tick) then 160/10 =16.
16 +1st tick = 17 ticks. 17 x 3hmp = 51mp. It is possible to reach this limit by resting immediately after the spell goes off.
Also note that Firestorm costs 30 mp to cast, so net gain is 21 mp base, 24 mp with Light Arts active, or 36 mp if Penury is used to halve the cost of the spell. This does not account for Conserve MP kicking in.
This is still a very good back piece for SCH as even without Penury, it would take a +1hmp back piece 24 ticks to match the mp recovered (with Light Arts active) and would require a +2hmp gear 18 ticks piece to match its efficiency when Penury is used without factoring in Conserve MP. Thus a a Scholar equipping the Casaba Melon Tank while casting Firestorm with Penury active is essentially wearing a +2hmp back piece for 3 minutes.